The research investigates the demographic causes and consequences of Tzotzil and Tzeltal Indian communities' participation in the Mexican land reform program since the 1930s. Archivally derived data on communities' subsistence farming acreage and increments thereto under Mexican land reform are being correlated with birth and death rates and changes therein to test a model in which features of Tzotzil and Tzeltal social organization are mediating links in feedback between subsistence patterns and population trends.